Epigenetics helps define current cell states, yet also shapes how cells respond to external cues such as differentiation or stress. The epigenetic plasticity of a cell describes how flexible this regulation is. Early embryonic cells are highly plastic in that they can generate all adult cell types. As development progresses, this plasticity is lost as normal healthy adult cells are locked in their identity. Crucially, aberrant reactivation may contribute to pathologies such as cancer.
Bivalent chromatin is an exemplar of epigenetic plasticity. This co-occurrence of the active-associated H3K4me3 and inactive-associated H3K27me3 histone modifications on opposite tails of the same nucleosome was first described in mouse embryonic stem cells where it is found at promoters of key developmental genes. It has been postulated that bivalent chromatin poises these promoters, keeping them free from repressive DNA methylation, enabling them to be transcriptionally upregulated upon differentiation. Bivalent chromatin has also been reported in other cell types including somatic cells and cancer cells, however we know little of the dynamics, resolution and regulation of this chromatin state. This is partly due to the technical challenges distinguishing bone-fide bivalent chromatin, where both marks are on the same nucleosome, from sample heterogeneity where some alleles have H3K4me3 and others H3K27me3. We have recently developed robust and sensitive methods to accurately profile bivalent chromatin in developmental and cancer models. I will present this work, along with our discovery of epigenetic priming factors responsible for establishing bivalent chromatin, and their roles in both developmental and cancer contexts.